The present invention relates to a polarizer in which cholesteric liquid crystal polymer layers are laminated with each other in a close contact state, which is thin in thickness, improves luminance and the angle of visibility of liquid crystal displays and the like, and inhibits changes in color with visual angle changes, and a lighting device and a liquid crystal display using the same.
Conventionally, circularly polarized light separation layers each comprising two or more cholesteric liquid crystal polymer layers different in helical pitch which are adhered to each other through an adhesive layer have been known in JP-A-1-133003 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d). The lamination of cholesteric liquid crystal layers aims at the enlargement of the wavelength region of reflected light. That is to say, the wavelength (xcex) of light reflected from a cholesteric liquid crystal layer is represented by nopcosxcex8 less than xcex less than nepcosxcex8, based on the refractive indexes of ordinary light and extraordinary light by birefringence (no, ne) and the helical pitch (p), taking the angle of incidence as xcex8. The wavelength region xcex of the reflected light is narrower than that of visible light, so that light transmitted through a monolayer cholesteric liquid crystal layer and light reflected therefrom look colored, as called selective reflection or circularly polarized light dichromatism. Accordingly, different types of cholesteric liquid crystal polymer layers are laminated to enlarge the wavelength region of the reflected light, thereby forming circularly polarized light separation layers showing neutral tints.
However, the lamination of the cholesteric liquid crystal polymer layers by means of the adhesive layer has the problems of increasing the thickness and increasing the reflection loss due to the difference in refractive indexes in an adhesion interface. Further, the mere lamination of the cholesteric liquid crystal polymer layers only adds the wavelengths of the reflected light. Accordingly, in order to form circularly polarized light separation layers, for example, showing reflection characteristics over the entire region of visible light, it is usually necessary to combine three or more types of cholesteric liquid crystal polymer layers so that the wavelength region of the reflected light extends to the entire region of visible light.
On the other hand, means for improving the efficiency for light utilization in liquid crystal displays by polarizing light from light sources through circularly polarized light separation layers to realize bright display have been known in JP-A-59-127019, JP-A-61-122626, JP-A-63-121821, JP-A-3-45906, JP-A-6-324333, JP-A-7-35925 and JP-A-7-36025.
However, although light vertically (front) incident on circularly polarized light separation layers is transmitted as right (left) circularly polarized light without changes in color, light obliquely incident thereon has the problem that it is transmitted as elliptically polarized light to cause changes in color (coloring). Thus, when such circularly polarized light separation layers are applied to liquid crystal display layers, obliquely transmitted light suffers changes in color based on both the liquid crystal cells and the circularly polarized light separation layers. As a result, the changes in color are combined when visually observed, which causes great changes in color with visual angle changes, resulting in a decline in visibility.
It is therefore an object of the present invention to provide a polarizer which is thin in thickness, has no reflection loss caused by intervention of an adhesive layer, and has a circularly polarized light separation layer wide in the reflection wavelength region which does not give changes in color to vertically transmitted light and can inhibit changes in color of obliquely transmitted light, thus being able to form a liquid crystal display in which light is efficiently transmitted through a polarizing plate as neutral tints to provide an excellent efficiency for light utilization and changes in color caused by a liquid crystal cell can also be inhibited to give an excellent brightness and a wide angle of visibility.
According to the present invention, there is provided a polarizer comprising a circularly polarized light separation layer in which two or more cholesteric liquid crystal polymer layers are laminated with each other in a close contact state in a long-short order based on a center wavelength of reflected light, the circularly polarized light separation layer changing in helical pitch in the direction of thickness, a retardation layer arranged on the long-wavelength side of the center wavelength, and a polarizing plate containing a dichroic substance and disposed on the retardation layer, if desired and Necessary.
The present invention further provides a lighting device in which a surface light source is arranged on the circularly polarized light separation layer side of the polarizer, and a liquid crystal display in which the polarizer or the lighting device is arranged on the visual back face side of a liquid crystal cell, directing the retardation layer side thereof to the liquid crystal cell.
According to the present invention, the circularly polarized light separation layer is obtained in which reflection loss caused by intervention of an adhesive layer and an increase in thickness are avoided, the polarizer can be obtained in which vertically incident light is transmitted as circularly polarized light without changes in color and can be linearly polarized through the retardation layer, and elliptically polarized light changed in color by oblique transmission through the circularly polarized light separation layer is also compensated in color through the retardation layer, thereby providing light giving neutral tints when it is transmitted through the polarizing plate, and the liquid crystal display having a wide angle of visibility, producing no changes in color, and having an excellent brightness and a good visibility can be obtained.